<!DOCTYPE html>
<style>
  html,body,svg { height:100%; margin:0; padding:0; }
</style>
<body>
  <svg viewBox="0 0 20 20" xmlns="http://www.w3.org/2000/svg">
    <rect width="10" height="10">
      <animate attributeName="rx" values="0;10;0" dur="20s" repeatCount="indefinite" />
    </rect>
  </svg>
</body>